A wide variety of optical filters have been fabricated which selectively pass or block wavelengths of interest. Some conventional wide acceptance angle filters, usually referred to as color filters, are relatively broadband and have passbands of 50 nanometers and more. Another type of optical filter is generally referred to as an interference filter and functions to control the spectral composition of radiant energy partially by the effects of its interference. These filters are fabricated by depositing thin layers of dielectric materials, each with a different index of refraction, on a polished glass substrate and are well known and have been widely used in a good number of applications, see for example the article entitled "Interference Filters" The Optical Industry and Systems Directory (1984) pages E-65 through E-67. They can allow narrow passbands and high transmittance to the selected wavelength at normal incident. The filter is transmissive at shorter wavelengths at angles off the normal incident (the larger the angle the shorter the wavelength). Other classes of filters are characterized by birefringent filters, atomic resonance filters, and so forth. Generally, these filters tend to be overly complex and limited in applications, partially because they are expensive.
Thus, a continuing need exists in the state-of-the-art for an optic filter which is cost effective and has narrow band and wide acceptance angle.